Perforated flame holder with integrated sub-quench distance layer

ABSTRACT

A flame holder assembly includes a flame holder element and a flame shield element. The flame holder element has a first plurality of apertures extending through the flame holder element. The flame shield element has a second plurality of apertures extending through the flame shield element. Each of the second plurality of apertures has a lateral dimension that is no greater than a flame quenching distance. The flame shield element is positioned facing the flame holder element, and the flame holder assembly is configured such that fuel is supplied to the flame holder element via the second plurality of apertures of the flame shield element.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. ProvisionalPatent Application No. 62/274,032, entitled “PERFORATED FLAME HOLDERWITH INTEGRATED SUB-QUENCH DISTANCE LAYER,” filed Dec. 31, 2015 (docketnumber 2651-269-02); which, to the extent not inconsistent with thedisclosure herein, is incorporated by reference.

BACKGROUND

The term quenching distance can be defined as the critical diameter of atube, critical dimension of an opening, or critical distance between twoparallel plates, through which a flame will not propagate. In practice,the quenching distance depends upon a number of factors, including thecomposition of the fuel (e.g., methane, propane, particulate, etc.), thecomposition of the fuel stream, (e.g., ratio of fuel to O₂, proportionof diluents, etc.), the shape of the passage, the thermalcharacteristics of the surrounding material, etc. The determination ofthe quenching distance for a specific application is well understood inthe art.

SUMMARY

According to an embodiment, a flame holder assembly is provided,including a flame holder element and a flame shield element. The flameholder element has first and second faces lying opposite each other, anda first plurality of apertures extending through the flame holderelement between the first and second faces, each of the first pluralityof apertures having lateral dimensions greater than a flame quenchingdistance. The flame shield element has third and fourth faces lyingopposite each other, and a second plurality of apertures extendingthrough the flame shield element between the third and fourth faces.Each of the second plurality of apertures has a lateral dimension thatis no greater than the flame quenching distance. The flame shieldelement is positioned with the third face of the flame shield elementfacing the second face of the flame holder element.

According to an embodiment, each of the first plurality of aperturesextends substantially unobstructed between the first and second faces.

According to an embodiment, the flame holder element has a void fractionof at least 0.50. According to further embodiments, the flame holderelement has a void fraction that is, respectively, at least 0.60, atleast 0.70, at least 0.80, and equal to about 0.70.

According to an embodiment, an assembly support element is provided,configured to hold the perforated flame holder element and the flameshield element in a spaced-apart relationship.

According to another embodiment, the flame holder element and the flameshield element are positioned with the third face of the flame shieldelement in direct contact with the second face of the flame holderelement.

According to an embodiment, each of the second plurality of apertureshas a slot shape, extending laterally in the flame shield element adistance at least equal to a distance between two adjacent ones of thefirst plurality of apertures.

According to an embodiment, a length of each of the first plurality ofapertures is greater than a transverse dimension of the respective oneof the first plurality of apertures by a factor of at least 4. Accordingto further embodiments, the length of each of the first plurality ofapertures is greater than a transverse dimension of the respective oneof the first plurality of apertures by a factor of, respectively, atleast 4, 6, 8, 12, 16, 24, and 48.

According to an embodiment, a preheat structure is provided, configuredto apply thermal energy to the flame holder element, when activated.

According to an embodiment, a method of operation is provided, in whicha fuel stream is introduced to a perforated flame holder via a pluralityof passages formed in a shield element that is positioned between theperforated flame holder and a source of the fuel stream, each of thepassages having a transverse dimension that is no greater than a flamequenching distance for a fuel component of the fuel stream. A majorityof the fuel stream is combusted within a plurality of aperturesextending between first and second faces of the perforated flame holder.

According to an embodiment, a quantity of fuel is combusted, sufficientto produce at least 1 MBTUH/ft² of thermal energy. According to furtherembodiments, the quantity of fuel combusted is sufficient to produce,respectively, at least 1.5, 3, and 5 MBTUH/ft² of thermal energy.

According to an embodiment, the perforated flame holder has a voidfraction of at least 0.50. According to further embodiments, the flameholder element has a void fraction that is, respectively, at least 0.60,at least 0.70, at least 0.80, and equal to about 0.70.

According to an embodiment, a length of each of the plurality ofapertures in the perforated flame holder is greater than a transversedimension of the respective one of the plurality of apertures by afactor of at least 4. According to further embodiments, the length ofeach of the plurality of apertures is greater than a transversedimension of the respective one of the plurality of apertures by afactor of, respectively, at least 4, 6, 8, 12, 16, 24, and 48. Accordingto an embodiment, each of the plurality of apertures extends withoutobstruction between the first and second faces of in the perforatedflame holder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a burner system including a perforatedflame holder configured to hold a combustion reaction, according to anembodiment.

FIG. 2 is a side sectional diagram of a portion of the perforated flameholder of FIG. 1, according to an embodiment.

FIG. 3 is a flow chart showing a method for operating a burner systemincluding the perforated flame holder shown and described herein,according to an embodiment.

FIG. 4 is a side-sectional diagram of a perforated flame holderassembly, according to an embodiment.

FIGS. 5A-5C are diagrams showing a flame holder assembly, according toanother embodiment. FIG. 5A is a side sectional view of the flame holderassembly; FIG. 5B is an enlarged view of the portion of the flame holderassembly indicated at 5B in FIG. 5A; and FIG. 5C is a plan view of aportion of the flame holder assembly of FIG. 5A, as viewed from thedownstream side of the assembly.

FIGS. 6A and 6B are respective views of a flame holder assembly,according to another embodiment. FIG. 6A is a plan view of a portion ofthe flame holder assembly, as viewed from the upstream side of theassembly.

FIG. 6B is a perspective view of a portion of the flame holder assemblyof FIG. 6A, with additional portions cut-away to show further details.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented here.

FIG. 1 is a simplified diagram of a burner system 100 including aperforated flame holder 102 configured to hold a combustion reaction,according to an embodiment. As used herein, the terms perforated flameholder, perforated reaction holder, porous flame holder, and porousreaction holder shall be considered synonymous unless further definitionis provided. Experiments performed by the inventors have shown thatperforated flame holders 102 described herein can support cleancombustion. Specifically, in experimental use of systems ranging frompilot scale to full scale, output of oxides of nitrogen (NOx) wasmeasured to range from low single digit parts per million (ppm) down toundetectable (less than 1 ppm) concentration of NOx at the stack. Theseremarkable results were measured at 3% (dry) oxygen (O₂) concentrationwith undetectable carbon monoxide (CO) at stack temperatures typical ofindustrial furnace applications (1400-1600° F.). Moreover, these resultsdid not require any extraordinary measures such as selective catalyticreduction (SCR), selective non-catalytic reduction (SNCR), water/steaminjection, external flue gas recirculation (FGR), or other heroicextremes that may be required for conventional burners to even approachsuch clean combustion.

According to embodiments, the burner system 100 includes a fuel andoxidant source 103 disposed to output fuel and oxidant into a combustionvolume 104 to form a fuel and oxidant mixture 106. As used herein, theterms combustion volume, combustion chamber, furnace volume, and thelike shall be considered synonymous unless further definition isprovided. The perforated flame holder 102 is disposed in the combustionvolume 104 and positioned to receive the fuel and oxidant mixture 106.

FIG. 2 is a side sectional diagram 200 of a portion of the perforatedflame holder 102 of FIG. 1, according to an embodiment. Referring toFIGS. 1 and 2, the perforated flame holder 102 includes a perforatedflame holder body 108 defining a plurality of perforations 110 extendingsubstantially unobstructed through the body of the perforated flameholder, aligned to receive the fuel and oxidant mixture 106 from thefuel and oxidant source 103. As used herein, the terms perforation,pore, aperture, elongated aperture, and the like, in the context of theperforated flame holder 102, shall be considered synonymous unlessfurther definition is provided. The perforations 110 are configured tocollectively hold a combustion reaction 202 supported by the fuel andoxidant mixture 106.

The fuel can include hydrogen, a hydrocarbon gas, a vaporizedhydrocarbon liquid, an atomized hydrocarbon liquid, or a powdered orpulverized solid. The fuel can be a single species or can include amixture of gas(es), vapor(s), atomized liquid(s), and/or pulverizedsolid(s). For example in a process heater application, the fuel caninclude fuel gas or byproducts from the process that include CO,hydrogen (H₂), and methane (CH₄). In another application the fuel caninclude natural gas (mostly CH₄) or propane (C₃H₈). In anotherapplication, the fuel can include #2 fuel oil or #6 fuel oil. Dual fuelapplications and flexible fuel applications are similarly contemplatedby the inventors. The oxidant can include O₂ carried by air and/or caninclude another oxidant, either pure or carried by a carrier gas. Theterms oxidant and oxidizer shall be considered synonymous herein.

According to an embodiment, the perforated flame holder body 108 can bebound by an input face 112 disposed to receive the fuel and oxidantmixture 106, an output face 114 facing away from the fuel and oxidantsource 103, and a peripheral surface 116 defining a lateral extent ofthe perforated flame holder 102. The plurality of perforations 110,which are defined by the perforated flame holder body 108, extend fromthe input face 112 to the output face 114. The plurality of perforations110 can receive the fuel and oxidant mixture 106 at the input face 112.The fuel and oxidant mixture 106 can then combust in or near theplurality of perforations 110 and combustion products can exit theplurality of perforations 110 at or near the output face 114.

According to an embodiment, the perforated flame holder 102 isconfigured to hold a majority of the combustion reaction 202 within theperforations 110. For example, on a steady-state basis, more than halfthe molecules of fuel output into the combustion volume 104 by the fueland oxidant source 103 may be converted to combustion products betweenthe input face 112 and the output face 114 of the perforated flameholder 102. According to an alternative interpretation, more than halfof the heat output by the combustion reaction 202 may be output betweenthe input face 112 and the output face 114 of the perforated flameholder 102. Under nominal operating conditions, the perforations 110 canbe configured to collectively hold at least 80% of the combustionreaction 202 between the input face 112 and the output face 114 of theperforated flame holder 102. In some experiments, the inventors produceda combustion reaction that was apparently wholly contained in theperforations 110 between the input face 112 and the output face 114 ofthe perforated flame holder 102. According to an alternativeinterpretation, the perforated flame holder 102 can support combustionbetween the input face 112 and output face 114 when combustion is“time-averaged.” For example, during transients, such as before theperforated flame holder 102 is fully heated, or if too high a (cooling)load is placed on the system, the combustion may travel somewhatdownstream from the output face 114 of the perforated flame holder 102.

While a “flame” is described in a manner intended for ease ofdescription, it should be understood that in some instances, no visibleflame is present. Combustion occurs primarily within the perforations110, but the “glow” of combustion heat is dominated by a visible glow ofthe perforated flame holder 102 itself, produced by the visible portionof thermal radiation 204 emitted during operation. In other instances,the inventors have noted transient “huffing” wherein a visible flamemomentarily ignites in a region lying between the input face 112 of theperforated flame holder 102 and a fuel source—a fuel nozzle 118, in theembodiment shown—within the dilution region D_(D). Such transienthuffing is generally short in duration such that, on a time-averagedbasis, a majority of combustion occurs within the perforations 110 ofthe perforated flame holder 102, between the input face 112 and theoutput face 114. In still other instances, the inventors have notedapparent combustion occurring above the output face 114 of theperforated flame holder 102, but still a majority of combustion occurredwithin the perforated flame holder 102 as evidenced by the continuedvisible glow (a visible wavelength tail of blackbody radiation) from theperforated flame holder 102.

The inventors have determined that the perforated flame holder 102 doesnot require turbulence-producing elements to hold combustion within theapertures 110, even during operation at surprisingly high output loads.The inventors have conducted tests in which output loads of up to about5 MBTUH/ft² were achieved, while holding the combustion reaction 202substantially within the apertures 110.

The perforated flame holder 102 can be configured to receive heat fromthe combustion reaction 202 and output a portion of the received heat asthermal radiation 204 to heat-receiving structures (e.g., furnace wallsand/or radiant section working fluid tubes) in or adjacent to thecombustion volume 104. As used herein, terms such as thermal radiation,infrared radiation, radiant heat, heat radiation, etc. are to beconstrued as being substantially synonymous, unless further definitionis provided. Specifically, such terms refer to blackbody radiation ofelectromagnetic energy, primarily in infrared wavelengths.

Referring especially to FIG. 2, the perforated flame holder 102 outputsanother portion of the received heat to the fuel and oxidant mixture 106received at the input face 112 of the perforated flame holder 102. Theperforated flame holder body 108 may receive heat from the (exothermic)combustion reaction 202 at least in heat receiving regions 206 ofperforation walls 208. Experimental evidence has suggested to theinventors that the position of the heat receiving regions 206, or atleast the position corresponding to a maximum rate of receipt of heat,can vary along the length of the perforation walls 208. In someexperiments, the location of maximum receipt of heat was apparentlybetween ⅓ and ½ of the distance from the input face 112 to the outputface 114 (i.e., somewhat nearer to the input face 112 than to the outputface 114). The inventors contemplate that the heat receiving regions 206may lie nearer to the output face 114 of the perforated flame holder 102under other conditions. Most probably, there is no clearly defined edgeof the heat receiving regions 206 (or for that matter, the heat outputregions 210, described below). For ease of understanding, the heatreceiving regions 206 and the heat output regions 210 will be describedas particular regions 206, 210.

The perforated flame holder body 108 can be characterized by a heatcapacity. The perforated flame holder body 108 may hold heat from thecombustion reaction 202 in an amount corresponding to the heat capacitytimes temperature rise, and transfer the heat from the heat receivingregions 206 to heat output regions 210 of the perforation walls 208.Generally, the heat output regions 210 are nearer to the input face 112than are the heat receiving regions 206. According to oneinterpretation, the perforated flame holder body 108 can transfer heatfrom the heat receiving regions 206 to the heat output regions 210 viathermal radiation, depicted graphically as 204. According to anotherinterpretation, the perforated flame holder body 108 can transfer heatfrom the heat receiving regions 206 to the heat output regions 210 viaheat conduction along heat conduction paths 212. The inventorscontemplate that both radiation and conduction heat transfer mechanismsmay be operative in transferring heat from the heat receiving regions206 to the heat output regions 210. In this way, the perforated flameholder 102 may act as a heat source to maintain the combustion reaction202, even under conditions where a combustion reaction would not bestable when supported from a conventional flame holder.

The inventors believe that the perforated flame holder 102 causes thecombustion reaction 202 to occur within thermal boundary layers 214formed adjacent to walls 208 of the perforations 110. As the relativelycool fuel and oxidant mixture 106 approaches the input face 112, theflow is split into portions that respectively travel through individualperforations 110. The hot perforated flame holder body 108 transfersheat to the fluid, notably within thermal boundary layers 214 thatprogressively thicken as more and more heat is transferred to theincoming fuel and oxidant mixture 106. After reaching a combustiontemperature (e.g. the auto-ignition temperature of the fuel), thereactants continue to flow while a chemical ignition delay time elapses,over which time the combustion reaction 202 occurs. Accordingly, thecombustion reaction 202 is shown as occurring within the thermalboundary layers 214. As flow progresses, the thermal boundary layers 214merge at a merger point 216. Ideally, the merger point 216 lies betweenthe input face 112 and output face 114 that defines the ends of theperforations 110. At some point, the combustion reaction 202 causes theflowing gas (and plasma) to output more heat to the body 108 than itreceives from the body 108. The heat is received at the heat receivingregion 206, is held by the body 108, and is transported to the heatoutput region 210 nearer to the input face 112, where the heat recyclesinto the cool reactants (and any included diluent) to raise them to thecombustion temperature.

In an embodiment, the plurality of perforations 110 are eachcharacterized by a length L defined as a reaction fluid propagation pathlength between the input face 112 and the output face 114 of theperforated flame holder 102. The reaction fluid includes the fuel andoxidant mixture 106 (optionally including nitrogen, flue gas, and/orother “non-reactive” species), reaction intermediates (includingtransition states in a plasma that characterizes the combustionreaction), and reaction products.

The plurality of perforations 110 can be each characterized by atransverse dimension D_(S) between opposing perforation walls 208. Theinventors have found that stable combustion can be maintained in theperforated flame holder 102 if the length L of each perforation 110 isat least four times the transverse dimension D_(S) of the perforation.In other embodiments, the length L can be greater than six times thetransverse dimension D_(S). For example, experiments have been run whereL is at least eight, at least twelve, at least sixteen, and at leasttwenty-four times the transverse dimension D_(S). Preferably, the lengthL is sufficiently long for thermal boundary layers 214 formed adjacentto the perforation walls 208 in a reaction fluid flowing through theperforations 110 to converge at merger points 216 within theperforations 110 between the input face 112 and the output face 114 ofthe perforated flame holder 102. In experiments, the inventors havefound L/D_(S) ratios between 12 and 48 to work well (i.e., produce lowNOx, produce low CO, and maintain stable combustion).

The perforated flame holder body 108 can be configured to convey heatbetween adjacent perforations 110. The heat conveyed between adjacentperforations 110 can be selected to cause heat output from thecombustion reaction portion 202 in a first perforation 110 to supplyheat to stabilize a combustion reaction portion 202 in an adjacentperforation 110.

Referring especially to FIG. 1, the fuel and oxidant source 103 canfurther include the fuel nozzle 118, configured to output fuel, and anoxidant source 120 configured to output a fluid including the oxidant.For example, the fuel nozzle 118 can be configured to output pure fuel.The oxidant source 120 can be configured to output combustion aircarrying oxygen.

The perforated flame holder 102 can be held by a perforated flame holdersupport structure 122 configured to hold the perforated flame holder 102a distance D_(D) away from the fuel nozzle 118. The fuel nozzle 118 canbe configured to emit a fuel jet selected to entrain the oxidant to formthe fuel and oxidant mixture 106 as the fuel jet and oxidant travelalong a path to the perforated flame holder 102 through a dilutiondistance D_(D) between the fuel nozzle 118 and the perforated flameholder 102. Additionally or alternatively (particularly when a blower isused to deliver oxidant combustion air), the oxidant or combustion airsource can be configured to entrain the fuel and the fuel and oxidanttravel through the dilution distance D_(D). In some embodiments, a fluegas recirculation path 124 can be provided. Additionally oralternatively, the fuel nozzle 118 can be configured to emit a fuel jetselected to entrain the oxidant and to entrain flue gas as the fuel jettravels through a dilution distance D_(D) between the fuel nozzle 118and the input face 112 of the perforated flame holder 102.

The fuel nozzle 118 can be configured to emit the fuel through one ormore fuel orifices 126 having a dimension that is referred to as “nozzlediameter.” The perforated flame holder support structure 122 can supportthe perforated flame holder 102 to receive the fuel and oxidant mixture106 at a distance D_(D) away from the fuel nozzle 118 greater than 20times the nozzle diameter. In another embodiment, the perforated flameholder 102 is disposed to receive the fuel and oxidant mixture 106 at adistance D_(D) away from the fuel nozzle 118 between 100 times and 1100times the nozzle diameter. Preferably, the perforated flame holdersupport structure 122 is configured to hold the perforated flame holder102 about 200 times the nozzle diameter or more away from the fuelnozzle 118. When the fuel and oxidant mixture 106 travels about 200times the nozzle diameter or more, the mixture is sufficientlyhomogenized to cause the combustion reaction 202 to output minimal NOx.

The fuel and oxidant source 103 can alternatively include a premix fueland oxidant source, according to an embodiment. A premix fuel andoxidant source can include a premix chamber (not shown), a fuel nozzleconfigured to output fuel into the premix chamber, and an air channelconfigured to output combustion air into the premix chamber. A flamearrestor can be disposed between the premix fuel and oxidant source andthe perforated flame holder 102 and be configured to prevent flameflashback into the premix fuel and oxidant source.

The combustion air source, whether configured for entrainment in thecombustion volume 104 or for premixing can include a blower configuredto force air through the fuel and air source 103.

The support structure 122 can be configured to support the perforatedflame holder 102 from a floor or wall (not shown) of the combustionvolume 104, for example. In another embodiment, the support structure122 supports the perforated flame holder 102 from the fuel and oxidantsource 103. Alternatively, the support structure 122 can suspend theperforated flame holder 102 from an overhead structure (such as a flue,in the case of an up-fired system). The support structure 122 cansupport the perforated flame holder 102 in various orientations anddirections.

The perforated flame holder 102 can include a single perforated flameholder body 108. In another embodiment, the perforated flame holder 102can include a plurality of adjacent perforated flame holder sectionsthat collectively provide a tiled perforated flame holder 102.

The perforated flame holder support structure 122 can be configured tosupport the plurality of perforated flame holder sections. Theperforated flame holder support structure 122 can include a metalsuperalloy, a cementatious, and/or ceramic refractory material. In anembodiment, the plurality of adjacent perforated flame holder sectionscan be joined with a fiber reinforced refractory cement.

The perforated flame holder 102 can have a width dimension W betweenopposite sides of the peripheral surface 116 at least twice a thicknessdimension T between the input face 112 and the output face 114. Inanother embodiment, the perforated flame holder 102 can have a widthdimension W between opposite sides of the peripheral surface 116 atleast three times, at least six times, or at least nine times athickness dimension T between the input face 112 and the output face 114of the perforated flame holder.

In an embodiment, the perforated flame holder 102 can have a widthdimension W less than a width of the combustion volume 104. This canallow the flue gas circulation path 124 from above to below theperforated flame holder 102 to lie between the peripheral surface 116 ofthe perforated flame holder 102 and the combustion volume wall (notshown).

Referring again to both FIGS. 1 and 2, the perforations 110 can includeelongated squares, each of the elongated squares having a transversedimension D_(S) between opposing sides of the squares. In anotherembodiment, the perforations 110 can include elongated hexagons, each ofthe elongated hexagons having a transverse dimension D_(S) betweenopposing sides of the hexagons. In another embodiment, the perforations110 can include hollow cylinders, each of the hollow cylinders having atransverse dimension D_(S) corresponding to a diameter of the cylinders.In another embodiment, the perforations 110 can include truncated cones,each of the truncated cones having a transverse dimension D_(S) that isrotationally symmetrical about a length axis that extends from the inputface 112 to the output face 114. The perforations 110 can each have alateral dimension D_(S) equal to or greater than a quenching distance ofthe fuel based on standard reference conditions.

In one range of embodiments, each of the plurality of perforations 110has a lateral dimension D_(S) between 0.05 inch and 1.0 inch.Preferably, each of the plurality of perforations 110 has a lateraldimension D_(S) between 0.1 inch and 0.5 inch. For example the pluralityof perforations 110 can each have a lateral dimension D_(S) of about 0.2to 0.4 inch.

The void fraction of a perforated flame holder 102 is defined as thetotal volume of all perforations 110 in a section of the perforatedflame holder 102 divided by a total volume of the perforated flameholder 102 including the body 108 and perforations 110. The perforatedflame holder 102 preferably has a void fraction between 0.10 and 0.90.In an embodiment, the perforated flame holder 102 can have a voidfraction between 0.30 and 0.80. In another embodiment, the perforatedflame holder 102 can have a void fraction of greater than about 0.50.According to another embodiment, the perforated flame holder 102 canhave a void fraction of greater than about 0.60. According to anotherembodiment, the perforated flame holder 102 can have a void fraction ofgreater than about 0.70. According to another embodiment, the perforatedflame holder 102 can have a void fraction of greater than about 0.80.According to a further embodiment, the perforated flame holder 102 canhave a void fraction of about 0.70. In experiments conducted by theinventors, a void fraction of about 0.70 was found to be especiallyeffective for producing very low NOx.

The perforated flame holder 102 can be formed from a fiber reinforcedcast refractory material and/or a refractory material such as analuminum silicate material. For example, the perforated flame holder 102can be formed from mullite or cordierite. Additionally or alternatively,the perforated flame holder body 108 can include a metal superalloy suchas Inconel® or Hastelloy®. The perforated flame holder body 108 candefine a honeycomb.

The inventors have found that the perforated flame holder 102 can beformed from VERSAGRID® ceramic honeycomb, available from AppliedCeramics, Inc. of Doraville, S.C.

The perforations 110 can be parallel to one another and normal to theinput and output faces 112, 114. In another embodiment, the perforations110 can be parallel to one another and formed at an angle relative tothe input and output faces 112, 114. In another embodiment, theperforations 110 can be non-parallel to one another. In anotherembodiment, the perforations 110 can be non-parallel to one another andnon-intersecting. In another embodiment, the perforations 110 can beintersecting. The body 108 can be one piece or can be formed from aplurality of sections.

In another embodiment, which is not necessarily preferred, theperforated flame holder 102 may be formed from reticulated fibers formedfrom an extruded ceramic material. The term “reticulated fibers” refersto a netlike structure.

In another embodiment, the perforated flame holder 102 can include aplurality of tubes or pipes bundled together. The plurality ofperforations 110 can include hollow cylinders and can optionally alsoinclude interstitial spaces between the bundled tubes. In an embodiment,the plurality of tubes can include ceramic tubes. Refractory cement canbe included between the tubes and configured to adhere the tubestogether. In another embodiment, the plurality of tubes can includemetal (e.g., superalloy) tubes. The plurality of tubes can be heldtogether by a metal tension member circumferential to the plurality oftubes and arranged to hold the plurality of tubes together. The metaltension member can include stainless steel, a superalloy metal wire,and/or a superalloy metal band.

The perforated flame holder body 108 can alternatively include stackedperforated sheets of material, each sheet having openings that connectwith openings of subjacent and superjacent sheets. The perforated sheetscan include perforated metal sheets, ceramic sheets and/or expandedsheets. In another embodiment, the perforated flame holder body 108 caninclude discontinuous packing bodies such that the perforations 110 areformed in the interstitial spaces between the discontinuous packingbodies. In one example, the discontinuous packing bodies includestructured packing shapes. In another example, the discontinuous packingbodies include random packing shapes. For example, the discontinuouspacking bodies can include ceramic Raschig ring, ceramic Berl saddles,ceramic Intalox® saddles, and/or metal rings or other shapes (e.g. SuperRaschig Rings) that may be held together by a metal cage.

The inventors contemplate various explanations for why burner systemsincluding the perforated flame holder 102 provide such clean combustion.

In one aspect, the perforated flame holder 102 acts as a heat source tomaintain a combustion reaction 202 even under conditions where acombustion reaction 202 would not be stable when supported by aconventional flame holder. This capability can be leveraged to supportcombustion using a leaner fuel-to-oxidant mixture than is typicallyfeasible. Thus, according to an embodiment, at the point where the fuelstream 106 contacts the input face 112 of the perforated flame holder102, an average fuel-to-oxidant ratio of the fuel stream 106 is below a(conventional) lower combustion limit of the fuel component of the fuelstream—lower combustion limit defines the lowest concentration of fuelat which a fuel/air mixture will burn when exposed to a momentaryignition source under normal atmospheric pressure and an ambienttemperature of 25° C. (77° F.).

According to one interpretation, the fuel and oxidant mixtures supportedby the perforated flame holder 102 may be more fuel-lean than mixturesthat would provide stable combustion in a conventional burner.Combustion near a lower combustion limit of fuel generally burns at alower adiabatic flame temperature than mixtures near the center of thelean-to-rich combustion limit range. Lower flame temperatures generallyevolve a lower concentration of NOx than higher flame temperatures. Inconventional flames, too-lean combustion is generally associated withhigh CO concentration at the stack. In contrast, the perforated flameholder 102 and systems including the perforated flame holder 102described herein were found to provide substantially complete combustionof CO (single digit ppm down to undetectable, depending on experimentalconditions), while supporting low NOx. In some embodiments, theinventors achieved stable combustion at what was understood to be verylean mixtures (that nevertheless produced only about 3% or lowermeasured O₂ concentration at the stack). Moreover, the inventors believeperforation walls 208 may act as a heat sink for the combustion fluid.This effect may alternatively or additionally reduce combustiontemperature.

According to another interpretation, production of NOx can be reduced ifthe combustion reaction 202 occurs over a very short duration of time.Rapid combustion causes the reactants (including oxygen and entrainednitrogen) to be exposed to NOx-formation temperature for a time tooshort for NOx formation kinetics to cause significant production of NOx.The time required for the reactants to pass through the perforated flameholder 102 is very short compared to a conventional flame. The low NOxproduction associated with perforated flame holder combustion may thusbe related to the short duration of time required for the reactants (andentrained nitrogen) to pass through the perforated flame holder 102.

Since CO oxidation is a relatively slow reaction, the time for passagethrough the perforated flame holder 102 (perhaps plus time passingtoward the flue from the perforated flame holder 102) is apparentlysufficient and at sufficiently elevated temperature, in view of the verylow measured (experimental and full scale) CO concentrations, foroxidation of CO to carbon dioxide (CO₂).

FIG. 3 is a flow chart showing a method 300 for operating a burnersystem including the perforated flame holder shown and described herein,according to an embodiment. To operate a burner system including aperforated flame holder, the perforated flame holder is first heated toa temperature sufficient to maintain combustion of the fuel and oxidantmixture.

According to a simplified description, the method 300 begins with step302, wherein the perforated flame holder is preheated to a start-uptemperature, T_(S). After the perforated flame holder is raised to thestart-up temperature, the method proceeds to step 304, wherein fuel andoxidant are provided to the perforated flame holder and combustion isheld by the perforated flame holder.

According to a more detailed description, step 302 begins with step 306,wherein start-up energy is provided at the perforated flame holder.Simultaneously or following providing start-up energy, a decision step308 determines whether the temperature T of the perforated flame holderis at or above the start-up temperature, T_(S). As long as thetemperature of the perforated flame holder is below its start-uptemperature, the method loops between steps 306 and 308 within thepreheat step 302. In step 308, if the temperature T of at least apredetermined portion of the perforated flame holder is greater than orequal to the start-up temperature, the method 300 proceeds to overallstep 304, wherein fuel and oxidant is supplied to and combustion is heldby the perforated flame holder.

Step 304 may be broken down into several discrete steps, at least someof which may occur simultaneously.

Proceeding from step 308, a fuel and oxidant mixture is provided to theperforated flame holder, as shown in step 310. The fuel and oxidant maybe provided by a fuel and oxidant source that includes a separate fuelnozzle and combustion air source, for example. In this approach, thefuel and combustion air are output in one or more directions selected tocause the fuel and combustion air mixture to be received by an inputface of the perforated flame holder. The fuel may entrain the combustionair (or alternatively, the combustion air may dilute the fuel) toprovide a fuel and oxidant mixture at the input face of the perforatedflame holder at a fuel dilution selected for a stable combustionreaction that can be held within the perforations of the perforatedflame holder.

Proceeding to step 312, the combustion reaction is held by theperforated flame holder.

In step 314, heat may be output from the perforated flame holder. Theheat output from the perforated flame holder may be used to power anindustrial process, heat a working fluid, generate electricity, orprovide motive power, for example.

In optional step 316, the presence of combustion may be sensed. Varioussensing approaches have been used and are contemplated by the inventors.Generally, combustion held by the perforated flame holder is very stableand no unusual sensing requirement is placed on the system. Combustionsensing may be performed using an infrared sensor, a video sensor, anultraviolet sensor, a charged species sensor, thermocouple, thermopile,and/or other known combustion sensing apparatuses. In an additional oralternative variant of step 316, a pilot flame or other ignition sourcemay be provided to cause ignition of the fuel and oxidant mixture in theevent combustion is lost at the perforated flame holder.

Proceeding to decision step 318, if combustion is sensed not to bestable, the method 300 may exit to step 324, wherein an error procedureis executed. For example, the error procedure may include turning offfuel flow, re-executing the preheating step 302, outputting an alarmsignal, igniting a stand-by combustion system, or other steps. If, instep 318, combustion in the perforated flame holder is determined to bestable, the method 300 proceeds to decision step 320, wherein it isdetermined if combustion parameters should be changed. If no combustionparameters are to be changed, the method loops (within step 304) back tostep 310, and the combustion process continues. If a change incombustion parameters is indicated, the method 300 proceeds to step 322,wherein the combustion parameter change is executed. After changing thecombustion parameter(s), the method loops (within step 304) back to step310, and combustion continues.

Combustion parameters may be scheduled to be changed, for example, if achange in heat demand is encountered. For example, if less heat isrequired (e.g., due to decreased electricity demand, decreased motivepower requirement, or lower industrial process throughput), the fuel andoxidant flow rate may be decreased in step 322. Conversely, if heatdemand is increased, then fuel and oxidant flow may be increased.Additionally or alternatively, if the combustion system is in a start-upmode, then fuel and oxidant flow may be gradually increased to theperforated flame holder over one or more iterations of the loop withinstep 304.

Referring again to FIG. 1, the burner system 100 includes a heater 128operatively coupled to the perforated flame holder 102. As described inconjunction with FIGS. 2 and 3, the perforated flame holder 102 operatesby outputting heat to the incoming fuel and oxidant mixture 106. Aftercombustion is established, this heat is provided by the combustionreaction 202; but before combustion is established, the heat is providedby the heater 128.

Various heating apparatuses have been used and are contemplated by theinventors. In some embodiments, the heater 128 can include a flameholder configured to support a flame disposed to heat the perforatedflame holder 102. The fuel and oxidant source 103 can include a fuelnozzle 118 configured to emit a fuel stream and an air source 120configured to output combustion air adjacent to the fuel stream. Thefuel nozzle 118 and air source 120 can be configured to output the fuelstream to be progressively diluted by the combustion air. The perforatedflame holder 102 can be disposed to receive a diluted fuel and airmixture 106 that supports a combustion reaction 202 that is stabilizedby the perforated flame holder 102 when the perforated flame holder 102is at an operating temperature. A start-up flame holder, in contrast,can be configured to support a start-up flame at a locationcorresponding to a relatively rich fuel and air mixture that is stablewithout stabilization provided by the heated perforated flame holder102.

The burner system 100 can further include a controller 130 operativelycoupled to the heater 128 and to a data interface 132. For example, thecontroller 130 can be configured to control a start-up flame holderactuator configured to cause the start-up flame holder to hold thestart-up flame when the perforated flame holder 102 needs to bepre-heated and to not hold the start-up flame when the perforated flameholder 102 is at an operating temperature (e.g., when T≧T_(S)).

Various approaches for actuating a start-up flame are contemplated. Inone embodiment, the start-up flame holder includes amechanically-actuated bluff body configured to be actuated to interceptthe fuel and oxidant mixture 106 to cause heat-recycling vortices andthereby hold a start-up flame; or to be actuated to not intercept thefuel and oxidant mixture 106 to cause the fuel and oxidant mixture 106to proceed to the perforated flame holder 102. In another embodiment, afuel control valve, blower, and/or damper may be used to select a fueland oxidant mixture flow rate that is sufficiently low for a start-upflame to be jet-stabilized; and upon reaching a perforated flame holder102 operating temperature, the flow rate may be increased to “blow out”the start-up flame. In another embodiment, the heater 128 may include anelectrical power supply operatively coupled to the controller 130 andconfigured to apply an electrical charge or voltage to the fuel andoxidant mixture 106. An electrically conductive start-up flame holdermay be selectively coupled to a voltage ground or other voltage selectedto attract the electrical charge in the fuel and oxidant mixture 106.The attraction of the electrical charge was found by the inventors tocause a start-up flame to be held by the electrically conductivestart-up flame holder.

In another embodiment, the heater 128 may include an electricalresistance heater 128 configured to output heat to the perforated flameholder 102 and/or to the fuel and oxidant mixture 106. The electricalresistance heater 128 can be configured to heat up the perforated flameholder 102 to an operating temperature. The heater 128 can furtherinclude a power supply and a switch operable, under control of thecontroller 130, to selectively couple the power supply to the electricalresistance heater 128.

An electrical resistance heater 128 can be formed in various ways. Forexample, the electrical resistance heater 128 can be formed fromKANTHAL® wire (available from Sandvik Materials Technology division ofSandvik AB of Hallstaham mar, Sweden) threaded through at least aportion of the perforations 110 formed by the perforated flame holderbody 108. Alternatively, the heater 128 can include an inductive heater,a high energy (e.g. microwave or laser) beam heater, a frictionalheater, or other types of heating technologies.

Other forms of start-up apparatuses are contemplated. For example, theheater 128 can include an electrical discharge igniter or hot surfaceigniter configured to output a pulsed ignition to the air and fuel.Additionally or alternatively, a start-up apparatus can include a pilotflame apparatus disposed to ignite a fuel and oxidant mixture 106 thatwould otherwise enter the perforated flame holder 102. An electricaldischarge igniter, hot surface igniter, and/or pilot flame apparatus canbe operatively coupled to the controller 130, which can cause theelectrical discharge igniter or pilot flame apparatus to maintaincombustion of the fuel and oxidant mixture 106 in or upstream from theperforated flame holder 102 before the perforated flame holder 102 isheated sufficiently to maintain combustion.

The burner system 100 can further include a sensor 134 operativelycoupled to the controller 130. The sensor 134 can include a heat sensorconfigured to detect infrared radiation or a temperature of theperforated flame holder 102. The controller 130 can be configured tocontrol the heating apparatus 128 responsive to input from the sensor134. Optionally, a fuel control valve 136 can be operatively coupled tothe controller 130 and configured to control a flow of fuel to the fueland oxidant source 103. Additionally or alternatively, an oxidant bloweror damper 138 can be operatively coupled to the controller 130 andconfigured to control flow of the oxidant (or combustion air).

The sensor 134 can further include a combustion sensor operativelycoupled to the control circuit 130, the combustion sensor beingconfigured to detect a temperature, video image, and/or spectralcharacteristic of a combustion reaction 202 held by the perforated flameholder 102. The fuel control valve 136 can be configured to control aflow of fuel from a fuel source to the fuel and oxidant source 103. Thecontroller 130 can be configured to control the fuel control valve 136responsive to input from the combustion sensor 134. The controller 130can be configured to control the fuel control valve 136 and/or oxidantblower or damper 138 to control a preheat flame type of heater 128 toheat the perforated flame holder 102 to an operating temperature. Thecontroller 130 can similarly control the fuel control valve 136 and/orthe oxidant blower or damper 138 to change the fuel and oxidant mixture106 flow responsive to a heat demand change received as data via thedata interface 132.

Hereafter, and in the claims, unless otherwise defined or limited, termssuch as fuel, fuel stream, fuel supply, etc., are to be construedbroadly as reading on a substance that includes fuel, but that can alsoinclude additional components, such as oxidant, recirculated combustionproducts, diluents, inert gases, ambient air, etc.

FIG. 4 is a side-sectional view of a perforated flame holder assembly400, according to an embodiment. The flame holder assembly 400 includesa flame holder element 402 and a flame shield element 404, held in aspaced-apart relationship by an assembly support element 406. The flameholder element 402 is structured substantially as described withreference to the perforated flame holder 102 of FIGS. 1 and 2, andincludes input and output faces 112, 114, and a plurality of apertures110.

The flame shield element 404 includes an upstream face 412 and adownstream face 414, and a plurality of fuel passages 408 extendingbetween the upstream and downstream faces 412, 414. Lateral dimensionsD_(P) of the fuel passages 408 are selected to be no larger than aquenching distance of a flame. As with the apertures 110 of the flameholder element 402, the fuel passages 408 can have any of a number ofshapes—in a transverse sectional view—including, for example, square,rectangular, round, and polygonal. Lateral dimensions of the flameshield element 404 are preferably coextensive with those of the flameholder element 402. In the embodiment of FIG. 4, the upstream face 412of the flame shield element 404 defines an input face 410 of the flameholder assembly 400.

According to an embodiment, the assembly support element 406 extendsaround the entire lateral perimeter of the flame holder assembly 400.This serves to prevent the introduction of fuel or other fluids into theassembly except via the fuel passages 408. This is of particular benefitin combustion systems in which the flame holder assembly 400 isconfigured to be held away from side walls of a combustion volume.

According to an embodiment, the flame holder element 402 can bepreheated prior to normal operation, in order to initiate and sustainthe combustion reaction 202. For example, an electrically resistiveheater 128 can be provided, positioned in close proximity, or in contactwith the flame holder element 402. Just prior to the introduction offuel to the flame holder assembly 400, a voltage is applied to theheater 128, which raises the temperature of at least a portion of theflame holder element 402 to an operating temperature, which issufficient to initiate combustion. Various other structures and methodsfor preheating the flame holder element 402 are contemplated, includingmany of those previously described.

During normal operation of the flame holder assembly 400, according toan embodiment, fuel—including oxidizer—is introduced to the flame holderassembly 400 via the fuel passages 408 of the flame shield element 404.A combustion reaction 202 is held within the apertures 110 of the flameholder 102 as explained in detail above, with reference to FIGS. 1 and2. Thermal radiation 204 is emitted by the flame holder assembly 400,particularly from the output face 114 of the flame holder element 402.Because the fuel passages 408 of the flame shield element 404 aresmaller than the quenching distance, the combustion reaction 202 cannotpass through the flame shield element 404, which reduces or eliminatesthe danger of flashback, in embodiments where this may be a concern,such as in cases where the fuel-to-oxidant ratio of the fuel stream 106is above the lower combustion limit as it is introduced to the inputface 410 of the flame holder assembly 400.

According to another embodiment, at the point where the fuel stream 106contacts the flame holder element 402, the average fuel-to-oxidant ratioof the fuel stream 106 is below the lower combustion limit of the fuelcomponent of the fuel stream, and only becomes flammable when it isheated by the combustion reaction 202. Even though flashback may not bean issue in such cases, the flame shield element 404 can be beneficialin other ways. For example, the flame shield element 404 may also serveto substantially block the radiation of thermal energy 204 from theflame holder element 402 in the upstream direction (i.e., toward thefuel source, etc.). This is beneficial, for example, in reducing theheat applied to system components located on the upstream side of theflame holder assembly 400 such as, e.g., fuel nozzles, valves, supportstructures, enclosures, etc. Furthermore, by substantially preventingthe radiation of thermal energy 204 in the upstream direction, a greaterproportion of the total thermal energy produced by the combustionreaction 202 is radiated downstream from the flame holder assembly 400.Thus—assuming a thermal load positioned in the downstream direction—fora given expenditure of fuel, more thermal energy is delivered to theload, or, alternatively, for a given value of thermal energy to bedelivered, the fuel expenditure can be reduced, as compared to a systemin which the flame shield is omitted.

According to an embodiment, the maximum thermal output of the flameholder assembly 400 is between 1 and 5 MBTUH/ft². According to anembodiment, the maximum thermal output of the flame holder assembly 400is greater than 1.5 MBTUH/ft². According to another embodiment, themaximum thermal output of the flame holder assembly 400 is greater than3 MBTUH/ft². According to a further embodiment, the maximum thermaloutput of the flame holder assembly 400 is greater than 5 MBTUH/ft².

It will be understood that the thermal output of the flame holderassembly 400 is directly related to the volume of fuel—includingoxidizer—that is supplied to the flame holder element 402. Because thedimensions D_(P) of the fuel passages 408 are relatively small, i.e.,smaller than the quenching distance, the void fraction of the shieldelement 404 may be lower than that of the flame holder element 402. Thiscan result in a significant pressure drop across the shield element 404,particularly in embodiments in which the number of passages 408 issubstantially equal to the number of apertures 110 in the flame holderelement 402 (see, e.g., FIG. 5). This, in turn may necessitate anincreased fuel pressure on the upstream side of the flame shield element404, depending upon the desired thermal output.

According to the embodiment shown in FIG. 4, the number of fuel passages408 per unit of transverse area is greater than the number of apertures110 per unit of area. This results in an increased void fraction of theflame shield element 404, which reduces the pressure required for agiven volume of fuel to pass through the flame shield element 404.According to another embodiment, the number of fuel passages 408 perunit of transverse area is selected to provide a void fractionsubstantially equal to that of the flame holder element 402. Accordingto a further embodiment, the number of fuel passages 408 per unit oftransverse area is selected to provide a void fraction that is greaterthan that of the flame holder element 402.

FIG. 4 shows an embodiment in which a fuel stream 106 is introduced tothe flame holder assembly 400 as from a fuel nozzle, substantially asdescribed with reference to FIG. 1. In embodiments in which a nozzle isemployed, the fuel and oxidizer can be premixed prior to passing throughthe nozzle, or the system can be configured such that the fuel stream106 entrains the oxidizer as it traverses the distance between thenozzle and the flame holder assembly 400. According to anotherembodiment, fuel is supplied via a manifold or mixing chamber coupled tothe input face 410 of the flame holder assembly 400. In suchembodiments, the oxidizer can be separately introduced to a mixingchamber, for example, where it then mixes with the fuel. Alternatively,the fuel can be premixed with the oxidizer prior to introduction into amanifold.

The flame shield element 404 can be manufactured using the same types ofmaterials that can be employed to make the flame holder element 402,including various ceramics and alloys, as previously discussed.Likewise, similar manufacturing processes can also be employed,including, for example, extrusion, casting, sintering, machining, andisostatic pressing. However, it should be noted that it is not essentialthat the flame holder element 402 and the flame shield element 404 bemade of the same material. In some embodiments, it may be advantageousfor one or more specific characteristics of the flame shield element 404to differ from those of the flame holder element 402. Suchcharacteristics may include, for example, thermal conductivity, thermalemissivity, thermal capacity, mechanical strength and toughness,coefficient of thermal expansion, etc.

FIGS. 5A-5C are diagrams showing a flame holder assembly 500, accordingto an embodiment. FIG. 5A is a side sectional view of the flame holderassembly 500; FIG. 5B is an enlarged view of the portion of the flameholder assembly 500 indicated at FIG. 5B of FIG. 5A; and FIG. 5C is aplan view of a portion of the flame holder assembly 500, as viewed fromthe downstream side of the assembly.

Referring to FIGS. 5A-5C, the flame holder assembly 500 includes a flameholder element 402 and a flame shield element 404, positioned inface-to-face contact with each other, i.e., with the downstream face 414of the flame shield element 404 arranged to be very close to, or indirect physical contact with the input face 112 of the flame holderelement 402. The flame shield element 404 of the flame holder assembly500 is configured such that there is a fuel passage 408 in alignmentwith each of the apertures 110 of the flame holder element 402, as shownin FIG. 5C. Fuel entering one of the fuel passages 408 is introduceddirectly into a corresponding one of the apertures 110 of the flameholder element 402.

Operation of the flame holder assembly 500 is substantially similar tothe operation described with reference to the flame holder assembly 400of FIG. 4. Because the dimensions D_(P) of the fuel passages 408 are nogreater than the quenching distance, fuel traverses the passages at arelatively high velocity, particularly under high heat outputconditions. This helps to cool the flame shield element 404, even inclose contact with the flame holder element 402. Accordingly, no morethan a small fraction of the thermal energy generated is emitted in theupstream direction.

According to an embodiment, the flame holder element 402 and flameshield element 404 are held in their relative positions by an assemblysupport element similar to the assembly support element 406 shown anddescribed with reference to FIG. 4. According to another embodiment, arefractory adhesive or cement is used to hold the flame holder element402 and flame shield element 404 in face-to-face contact. According to afurther embodiment, the flame holder element 402 and flame shieldelement 404 are simply stacked together and supported by a bracketwithin a furnace, the support bracket being configured to preventlateral movement of either element during operation, to preventmisalignment of the flame holder element 402 and the flame shieldelement 404.

According to an embodiment, the flame holder element 402 and flameshield element 404 are formed together in a single piece. This can beachieved by forming the flame holder assembly 500 in a castingoperation, or by machining the assembly from a single block of material,etc. In embodiments in which the flame holder element 402 and flameshield element 404 are formed together in a single piece, the downstreamface 414 of the flame shield element 404 and the input face 112 of theflame holder element 402 can be considered to be merged, and defined bythe positions within the single piece where the apertures 110 transitionto fuel passages 408. It should be noted that in some embodiments, eachof the transitions may not lie in a common plane, inasmuch as thetransitions may be somewhat beveled, and/or the depths of the positionsmay vary.

FIGS. 6A and 6B are respective views of a flame holder assembly 600,according to an embodiment. FIG. 6A is a plan view of a portion of theflame holder assembly 600, as viewed from the input face 410 of theassembly. FIG. 6B is a perspective view of a portion of the flame holderassembly 600, with portions cut-away to show further details.

Referring to FIGS. 6A and 6B, the flame holder assembly 600 includes aflame holder element 402 and a flame shield element 602, positioned inface-to-face contact with each other. The flame shield element 602includes a plurality of fuel passages 604 having a slot shape, eachextending to admit fuel to a plurality of the apertures 110 in the flameholder element 402. According to an embodiment, the slot-shaped fuelpassages 604 have a width D_(P) that is no greater than the quenchingdistance.

According to an embodiment, the flame shield element 602 is held in aface-to-face relationship with the flame holder element 402 by anassembly support element 606 similar to the assembly support element 406shown and described with reference to FIG. 4. According to anotherembodiment, a refractory adhesive or cement is used to hold the flameholder element 402 and flame shield element 602 in face-to-face contact.According to a further embodiment, the flame holder element 402 andflame shield element 602 are stacked together and held by a supportbracket within a furnace. According to another embodiment, the flameholder element 402 and flame shield element 602 are formed together in asingle piece.

According to an embodiment, the flame shield element 602 is spaced apartfrom the flame holder element 402, similar to the configuration of theflame holder assembly 400 described with reference to FIG. 4. Inembodiments in which the flame shield element 602 and the flame holderelement 402 are spaced apart, the fuel passages 604 can be spaced at apitch that is different from the pitch of rows of apertures 110 in theflame holder element 402, i.e., there is no necessary correspondencebetween one of the fuel passages and a row of apertures. Thus, the voidfraction of the flame shield element 602 can be increased by increasingthe number of slot-shaped fuel passages 604, with a correspondingreduction in the distance between the passages.

One benefit provided by embodiments that include slot-shaped fuelpassages 604 like those described above is that, for a given row pitch,the void fraction of the shield element 602 can be much greater than thevoid fraction of a shield element 602 that has fuel passages 604 likethose described above with reference to FIGS. 4 and 5, i.e., square orround, etc. The increased void fraction produces a significantly lowerpressure drop in the fuel as it traverses the shield element 602. This,in turn, means that, for a given output, the fuel pressure required onthe input side of the flame holder assembly 500 is also reduced. Thishas an impact on the design parameters and cost of structures such asfuel pumps, blowers, nozzles, etc. Furthermore, in embodiments in whicha flame holder assembly 500 is spaced away from side walls defining acombustion volume 104, a lower input pressure reduces the likelihoodthat some fraction of the fuel stream 106 will pass around, rather thanthrough, the flame holder assembly 500.

Ordinal numbers, e.g., first, second, third, etc., are used in theclaims according to conventional claim practice, i.e., for the purposeof clearly distinguishing between claimed elements or features thereof.The use of such numbers does not suggest any other relationship, e.g.,order of operation or relative position of such elements. Furthermore,an ordinal number used to refer to an element in the claims does notnecessarily correlate to a number used in the specification to refer toan element of a disclosed embodiment on which those claims read, nor tonumbers used in unrelated claims to designate similar elements orfeatures.

The abstract of the present disclosure is provided as a brief outline ofsome of the principles of the invention according to one embodiment, andis not intended as a complete or definitive description of anyembodiment thereof, nor should it be relied upon to define terms used inthe specification or claims. The abstract does not limit the scope ofthe claims.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments are contemplated, including systems in whichelements of one or more of the disclosed embodiments are combined withelements that are known in the art, to provide additional embodiments.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A flame holder assembly, comprising: a flameholder element, including a first face and a second face lying oppositeeach other, and a first plurality of apertures extending through theflame holder element between the first and second faces, each of thefirst plurality of apertures having lateral dimensions that are greaterthan a flame quenching distance, the flame holder element having a voidfraction greater than 0.50; and a flame shield element including a thirdface and a fourth face lying opposite each other, and a second pluralityof apertures extending through the flame shield element between thethird and fourth faces, each of the second plurality of apertures havingat least one lateral dimension that is no greater than the flamequenching distance, the flame shield element being positioned with thethird face facing the second face of the flame holder element.
 2. Theflame holder assembly of claim 1, wherein the flame holder element isconfigured to hold a combustion reaction substantially within the firstplurality of apertures and between the first and second faces.
 3. Theflame holder assembly of claim 2, wherein each of the first plurality ofapertures extends substantially unobstructed between the first andsecond faces.
 4. The flame holder assembly of claim 1, comprising anassembly support element configured to hold the flame holder element andthe flame shield element in a spaced-apart relationship.
 5. The flameholder assembly of claim 4, wherein the assembly support element extendsaround an entire lateral perimeter of the flame holder assembly,enclosing a space between the flame holder element and the flame shieldelement.
 6. The flame holder assembly of claim 1, wherein the flameholder element and flame shield element are positioned with the thirdface of the flame shield element in direct contact with the second faceof the flame holder element.
 7. The flame holder assembly of claim 1,wherein each of the second plurality of apertures has a slot shape,extending laterally in the flame shield element a distance at leastequal to a distance between two adjacent ones of the first plurality ofapertures.
 8. The flame holder assembly of claim 1, wherein the flameholder element has a void fraction of greater than 0.50.
 9. The flameholder assembly of claim 8, wherein the flame holder element has a voidfraction of greater than 0.60.
 10. The flame holder assembly of claim 8,wherein the flame holder element has a void fraction of about 0.70. 11.The flame holder assembly of claim 1, wherein a length of each of thefirst plurality of apertures is greater than a transverse dimension ofthe respective one of the first plurality of apertures by a factor of atleast
 4. 12. The flame holder assembly of claim 11, wherein the lengthof each of the first plurality of apertures is greater than a transversedimension of the respective one of the first plurality of apertures by afactor of at least
 12. 13. The flame holder assembly of claim 12,wherein the length of each of the first plurality of apertures isgreater than a transverse dimension of the respective one of the firstplurality of apertures by a factor of at least
 16. 14. The flame holderassembly of claim 13, wherein the length of each of the first pluralityof apertures is greater than a transverse dimension of the respectiveone of the first plurality of apertures by a factor of at least
 24. 15.The flame holder assembly of claim 14, wherein the length of each of thefirst plurality of apertures is greater than a transverse dimension ofthe respective one of the first plurality of apertures by a factor of atleast
 48. 16. The flame holder assembly of claim 1, comprising a preheatstructure configured to apply thermal energy to the flame holderelement, when activated.
 17. The flame holder assembly of claim 16,wherein the preheat structure includes an electrical heating elementpositioned adjacent to the flame holder element.
 18. A method,comprising: introducing a fuel stream to a perforated flame holderhaving a void fraction of at least 0.50, via a plurality of passagesformed in a shield element positioned between the perforated flameholder and a source of the fuel stream, each of the passages havingtransverse dimensions that are no greater than a quenching distance fora fuel component of the fuel stream; and combusting a majority of thefuel within a plurality of apertures extending between first and secondfaces of the perforated flame holder.
 19. The method of claim 18,wherein the introducing a fuel stream to a perforated flame holderhaving a void fraction of at least 0.50 comprises introducing a fuelstream to a perforated flame holder having a void fraction of at least0.60.
 20. The method of claim 18, wherein the introducing a fuel streamto a perforated flame holder having a void fraction of at least 0.50comprises introducing a fuel stream to a perforated flame holder havinga void fraction of about 0.70.
 21. The method of claim 18, wherein theintroducing a fuel stream to a perforated flame holder comprisesintroducing a fuel stream having an average fuel-to-oxidant ratio thatis below a lower combustion limit of the fuel component of the fuelstream.
 22. The method of claim 18, comprising premixing the fuelstream, including adding a fuel component to an oxidant component. 23.The method of claim 18, wherein the combusting a majority of the fuelwithin a plurality of apertures extending between first and second facesof the perforated flame holder comprises combusting a majority of thefuel stream within a plurality of apertures that extend withoutobstruction between the first and second faces of the perforated flameholder.
 24. The method of claim 23, comprising combusting a quantity offuel sufficient to produce at least 1.5 MBTUH/ft² of thermal energy. 25.The method of claim 24, wherein the combusting a quantity of fuelsufficient to produce at least 1.5 MBTUH/ft² of thermal energy comprisescombusting a quantity of fuel sufficient to produce at least 3 MBTUH/ft²of thermal energy.
 26. The method of claim 25, wherein the combusting aquantity of fuel sufficient to produce at least 3 MBTUH/ft² of thermalenergy comprises combusting a quantity of fuel sufficient to produce atleast 5 MBTUH/ft² of thermal energy.
 27. The method of claim 23,comprising preheating at least a portion of the perforated flame holderto a start-up temperature during a start-up procedure.
 28. The method ofclaim 27, wherein the preheating at least a portion of the perforatedflame holder comprises applying an electrical current to an electricallyresistive element positioned adjacent to the perforated flame holder.29. A method, comprising: introducing a fuel stream to a perforatedflame holder via a plurality of passages formed in a shield elementpositioned between the perforated flame holder and a source of the fuelstream, each of the passages having a transverse dimension that is nogreater than a quenching distance for a fuel component of the fuelstream; and combusting a majority of the fuel within a plurality ofapertures extending between first and second faces of the perforatedflame holder, including combusting a quantity of fuel sufficient toproduce at least 1.5 MBTUH/ft².
 30. The method of claim 29, wherein thecombusting a quantity of fuel sufficient to produce at least 1.5MBTUH/ft² comprises combusting a quantity of fuel sufficient to produceat least 3 MBTUH/ft².
 31. The method of claim 30, wherein the combustinga quantity of fuel sufficient to produce at least 3 MBTUH/ft² comprisescombusting a quantity of fuel sufficient to produce at least 5MBTUH/ft².